Blog Country Specific Expat Tax Situations

UK Self Assessment Tax Returns: A Guide for US Expats

Share via

If you’re living and working in the UK, Self Assessment tax returns are essential to staying compliant with HMRC. Whether you’re self-employed, earning foreign income, or filing taxes in both the US and UK, this guide explains who needs to file, how the process works, and what it costs.

Filing Both US and UK Taxes? We Can Help

At Greenback, we don’t just help Americans file US expat taxes. We also prepare UK Self Assessment tax returns. If you live in the UK, our team can handle both your US and UK filings so you stay fully compliant. Learn more about Greenback’s UK tax services →

What Is Self Assessment in the UK?

Self Assessment is the UK’s system for reporting income that hasn’t been taxed at source. Unlike employees who have tax automatically deducted through PAYE (Pay As You Earn), Self Assessment requires you to report your income, calculate what you owe, and file a tax return directly with HMRC.

The UK tax year runs from April 6 to April 5 (not the calendar year), and Self Assessment returns cover all taxable income earned during that period.

Who Needs to File a Self Assessment Tax Return?

You’re required to file a UK Self Assessment tax return if any of the following apply:

You’re self-employed and earned more than £1,000
You’re a partner in a business partnership
You earn £100,000 or more in total taxable income
You or your partner earns £50,000+ and claims Child Benefit
You earn more than £2,500 in untaxed income (including tips, rental income, or foreign income)
You’re a company director (unless you received no compensation at all)
You have untaxed income from savings, investments, or dividends over £10,000
You owe Capital Gains Tax from selling assets
You earn a UK income while living abroad
You receive foreign income that needs to be reported to HMRC

The rules can be complex and situation-dependent. If you’re unsure whether you need to file, consult an expat tax professional who is familiar with both the UK and US tax systems.

For Americans in the UK: You may need to file a Self Assessment even if your income is already reported to the IRS. Foreign income thresholds and dual-country filing requirements often trigger Self Assessment obligations.

When Do I Register for Self Assessment?

If you’ve never filed a Self Assessment return before, you must register with HMRC first.

Registration deadlines:

If you’re self-employed: Register by October 5, following the end of the tax year
If you need to file for another reason, register as soon as you know you need to file

Once registered, HMRC will send you a Unique Taxpayer Reference (UTR) number, which you’ll need to file your return. The UTR can take up to 10 business days to arrive by post (or up to 21 days if you’re overseas).

You’ll also need to set up a Government Gateway account to file online, which requires an activation code sent by mail.

Learn more about registering for Self Assessment on the GOV.UK website →

UK Self Assessment Deadlines

The deadline for filing your Self Assessment tax return depends on how you file:

Paper returns: October 31, following the end of the tax year
Online returns: January 31, following the end of the tax year

Payment deadlines:

First payment on account (if applicable): January 31
Second payment on account: July 31
Final balance: January 31

UK vs US Tax Filing Deadlines

If you’re filing both UK and US taxes, here’s how the deadlines compare:

Country	Tax Year	Filing Deadline	Extension Deadline
UK	April 6 – April 5	January 31 (online)	None (late filing penalties apply)
US	January 1 – December 31	June 15 (automatic extension for expats, typical deadline is April 15)	October 15 (filing an extension request is required)

Why this matters for expats: UK Self Assessment is due in January, while US expat tax returns aren’t due until June 15 (with an extension to October 15). Many Americans in the UK file their UK return first, then use that information to complete their US return. Managing overlapping deadlines and exchange rate calculations requires careful planning.

How to File Your Self Assessment Tax Return

Once you’re registered and have your UTR number, there are three ways to file your Self Assessment return:

1. File Online Through Government Gateway

This is the most common method. After registering, you’ll receive an activation code by mail to complete your Government Gateway setup. You can then file directly through HMRC’s online portal.

2. Use Commercial Tax Software

HMRC-recognized software can simplify the filing process, especially if you have complex income sources. See the official list of approved software providers here.

3. File by Paper

You can download Form SA100, complete it by hand, and mail it to HMRC. Remember, paper returns have an earlier deadline (October 31).

How Much Does It Cost to File a Self Assessment?

If you’re an American in the UK filing both Self Assessment and US taxes, working with a specialized expat tax service offers significant advantages over hiring separate accountants in each country.

At Greenback, we’ve partnered with a UK Chartered Accountant to provide seamless coordination between your US and UK tax returns:

Single document upload: Submit your tax documents once through your secure Tax Companion account
Coordinated filing: Your US and UK accountants work together to ensure accurate, tax-efficient reporting across both countries
Specialized expertise: Our UK partner has deep experience with cross-border tax matters for Americans living in the UK
Flat-fee pricing: Transparent costs with no hidden charges

This collaborative approach saves time, reduces errors, and often results in better tax outcomes than working with separate accountants who don’t communicate with each other.

Why bundled services matter:

When your US and UK accountants work in silos, you risk:

Duplicate work explaining your situation twice
Inconsistent treatment of foreign income
Missed opportunities for tax optimization
Exchange rate calculation errors
Higher overall costs from coordination time

Learn more about our UK tax services and get a quote →

Other costs to consider:

Late filing penalties (see below)
Payment on account: If you owe more than £1,000 in tax, HMRC requires advance payments for the next year
Amended return fees: If errors are discovered after filing

Self Assessment Penalties and Late Filing Consequences

HMRC imposes automatic penalties if you file or pay late. Here’s what you’ll face:

Late Filing Penalties

Timing	Penalty
1 day late	£100 fixed penalty (even if no tax is owed)
3 months late	£10 per day for up to 90 days (£900 maximum)
6 months late	£300 or 5% of tax owed (whichever is higher)
12 months late	Additional £300 or 5% of tax owed, plus potential investigation

Late Payment Penalties

Timing	Penalty
30 days late	5% of tax owed
6 months late	An additional 5% of tax owed
12 months late	Additional 5% of tax owed

Interest charges: HMRC also charges interest on unpaid tax from the due date until payment is made. The current rate is set by HMRC and compounds daily.

Can penalties be appealed? Yes, if you have a reasonable excuse (such as serious illness, bereavement, or technical issues). HMRC reviews appeals on a case-by-case basis.

For expats: If you’re new to the UK tax system and genuinely didn’t know you needed to file, HMRC may waive first-time penalties, but only if you proactively register and file as soon as you realize your obligation.

Need help filing your UK Self Assessment and US tax return together?

At Greenback, we don’t just help Americans file US expat taxes — we also prepare UK Self Assessment tax returns. If you live in the UK, our team can handle both your US and UK filings so you stay fully compliant.

Making Tax Digital (MTD) for Self Assessment

Making Tax Digital is HMRC’s initiative to modernize tax reporting by requiring digital record-keeping and quarterly updates through compatible software.

Current status:

MTD has been mandatory for VAT-registered businesses since 2019
MTD for Income Tax Self Assessment is being rolled out in phases starting April 2026

Who will be affected:

Initially: Self-employed individuals and landlords with income over £50,000
Eventually: The threshold will likely drop to £30,000, then £10,000

What it means:

You’ll need to use MTD-compatible software to keep digital records
You’ll submit quarterly updates to HMRC instead of one annual return
Your annual Self Assessment filing will still be required, but much of the data will already be submitted

For Americans in the UK: MTD will add another layer of quarterly reporting alongside your existing US and UK annual filing obligations. Planning ahead and using compliant software will be essential.

Read our full guide to Making Tax Digital →

Need Help Filing Your UK Self Assessment and US Tax Return Together?

Dual filings involve two deadlines, overlapping rules, and complex foreign income reporting requirements. Greenback specializes in helping expats prepare both returns (including your UK Self Assessment) accurately and on time.

Our UK tax services include:

UK Self Assessment preparation and filing
Foreign income and Capital Gains reporting
Coordination with your US expat tax return
Exchange rate calculations and compliance

Filing taxes in the UK and the US doesn’t have to be complicated. Learn more about Greenback’s UK tax services →

What’s Different for Americans Filing UK Self Assessment?

As a US citizen or green card holder in the UK, you face additional requirements that UK-only filers don’t:

Dual filing obligations: You must file both UK Self Assessment and US tax returns, reporting worldwide income to both HMRC and the IRS.
Exchange rate conversions: Income and expenses must be converted using different rates for each country (spot rates for HMRC, annual averages for the IRS), which can create discrepancies between returns.
Overlapping deadlines: The UK Self Assessment is due on January 31, while US returns aren’t due until June 15 (or October 15 with an extension). Most expats file UK first, then use that data for their US return.
Treaty benefits to claim: The US-UK tax treaty prevents double taxation through Foreign Tax Credits and the Foreign Earned Income Exclusion (up to $130,000 for the 2025 tax year), but you must actively claim these benefits.

These complexities make professional help valuable. Read our full guide to US taxes for expats in the UK →

Get Started with UK and US Tax Services

Filing taxes in the UK and the US doesn’t have to be complicated. Greenback specializes in helping expats prepare both returns (including your UK Self Assessment) accurately and on time. Get started with UK tax services, or reach out with questions about your specific situation.

learn more about our UK tax services for expats →

Need help filing your UK Self Assessment and US tax return together?

Get Started Today

Frequently Asked Questions

Can I file my Self Assessment tax return late without incurring a penalty?

No. HMRC imposes automatic penalties starting from just one day late. However, if you have a reasonable excuse, you may be able to appeal the penalty.

Do I need to file a Self Assessment if I only have PAYE income?

Generally, no, unless you earn £100,000+ or meet one of the other criteria listed above. PAYE income is already taxed at source.

What happens if I don’t file a Self Assessment?

HMRC will issue penalties, charge interest on unpaid tax, and may open an investigation. Persistent non-compliance can result in court action.

How do I file a Self Assessment if I move abroad mid-year?

You’ll still need to file for the portion of the tax year you were a UK resident. You may also need to file a “split year” return depending on your circumstances.